FIG. 1 is a schematic block diagram of a communication system comprising a first user terminal UE1 and a second user terminal UE2. The first and second user terminals are in communication over a communication network 2. In order for communications to be provided between the first and second user terminals, a communication channel is set up between them. This channel comprises a first leg between the user terminal and the network, a leg across the network itself, and a leg between the network and the second user terminal. Each leg can be wired or wireless—wireless connections are shown by way of example. The communication network can be packet switched, that is data to be transmitted from the first user terminal to the second user terminal takes the form of packets to be conveyed over the communication channel set up between them.
In an attempt to try and improve data flow, minimise congestion and for efficiency's sake, rate control is widely used in such transmissions. That is, the rate at which data is transmitted from the sending side (transmitter) is controlled based on information relating to the available bandwidth over which it is to be transmitted. One approach to rate control is end-to-end rate control. End-to-end rate control can be implemented in a number of different forms, but in principle, relies on information about the channel which exists from end-to-end between the sending side (transmitters) and the receiving side (receiver). In any implementation of end-to-end rate control, a message from the transmitter has to reach the receiver, and information relating to the message (or the message itself) returned from the receiver to the transmitter in a round trip. End-to-end rate control has the advantage that no assumptions are made as to where the smallest bottleneck on the communication channel sits. On the other hand, as congestion starts building up at the bottleneck, this has to be detected at the receiver and a message returned to the sender. Thus, reacting to congestion takes at least a round-trip time (RTT), during which the congestion becomes worse, potentially harming a real time communication experience.